Screw extruders have been typically designed and used for conveying viscous polymeric materials. More recently, the ability of screw extruders to handle solids has become of interest for the purpose of expanding our usable energy resource base through gasification and liquifaction of coal and biomass. For example, U.S. Pat. No. 4,191,535 to Furman teaches an extrusion apparatus for pumping coal-containing particulate mixtures with the aid of a lubricant to reduce power requirements. Also, a paper entitled, "Extrusion of Concentrated Cellulosic/Polymer Slurries" by D. H. White and D. Wolf, presented to the Society of Plastics Engineers at the Annual Technical Meeting in New York, May 5-8, 1980, discusses the use of an extruder to convey sawdust slurries against 3000 psi systems.
However, until now it has not been appreciated how to efficiently pump low density, compressible bulk solids against high pressures, which is a capability necessary to achieve economical conversion of biomass-type feedstocks to liquid fuels.